Various types of rail guns have been proposed for using electromagnetic forces to accelerate projectiles to high velocities and direct them toward a target. U.S. Pat. Nos. 1,985,254; 4,624,173; 4,846,911; 5,076,135; 5,125,179; 5,285,763; 5,454,289 and 6,725,759; and British Patent Application No. 2,187,826A are examples of such guns which are made with barrels of different constructions.
A typical rail gun includes an elongated barrel which has a pair of longitudinally extending, parallel conductors or rails disposed symmetrically about its axis. The rails are separated by a pair of elongated insulating members and are connected at their rearward or breech ends to opposite terminals of a source of direct current. A circuit through the rails may be completed either by a conductor disposed between the rails or by a plasma arc between the rails and results in the flow of current which generates magnetic flux between the rails. The flux cooperates with the current in the conductor or the plasma to accelerate the conductor or plasma along between the rails. The projectile may include the conductor or may be positioned forward of the conductor or plasma arc and is driven forward thereby.
In addition to accelerating the projectile forward, electromagnetic forces generated during firing of the rail gun will include repulsion forces which tend to push the rails apart, i.e. outward. When a plasma arc is employed, additional bursting forces may result from gas pressure generated within the barrel which will act more symmetrically upon the entire bore structure. These radially outward forces are comparable, or greater in magnitude, to those experienced in barrels of conventional chemical explosive guns. Moreover, when a rail gun is fired, the rails conduct very high electrical current and are thus heated to high temperatures while being subjected to these bursting forces, so it may be desirable that the rails be cooled during firing.
In a rail gun, it is desirable that the two rails and the adjacent insulating members fit together with very close tolerances and be tightly constrained against displacement radially outward. It may also be desirable that the barrel be relatively light so that it may be moved rapidly for aiming, particularly if such is designed for shipboard use where it may be necessary to stabilize location for firing in a heavy sea. Relative lightness is also advantageous in minimizing the amount of droop or sag in an elongated, cantilevered structure such as a barrel for an artillery piece.
Rail gun barrel assemblies have employed various compression devices about the barrel components to resist these bursting forces. The '173 patent teaches the use of a pair of generally pie-shaped rails (in cross-section) that are spaced apart by a pair of generally pie-shaped insulators located within a rigid outer shell of circular cross-section. A pressure medium is employed to fill the annular part of the barrel between the interior surface of the shell and the exterior surface of the rail-insulator composite structure and supply a preload thereto. Examples of pressure media disclosed were water, oil, a resin that would harden in situ and continue to apply compressive stress, and an elastomeric material that would fill the annular region and then be pressurized, as by inward movement of screw pistons or the like.
The '135 patent describes certain more sophisticated ways of using a fluid-filled bladder to pressurize the region of a rail gun barrel surrounding a bore-defining composite structure located interior of a rigid, oval containment tube. The '289 patent injects an epoxy resin or the like under a pressure of 2000-4000 psi into such an annular surrounding region which is hardened to apply a compressively preload to the composite rail-insulator structure that defines the bore. The British application shows preloading a pair of rails within a tubular steel shell through the supply of hydraulic fluid to a pair of rubber bladders in elongated chambers of rectangular cross-section that are disposed immediately radially outward of the spaced-apart rails.
The '911 patent discloses compressive preloading of a rail-insulator composite structure that defines the bore by employing an oval shape, filament-wound shell and a pair of spacers of rigid ceramic material. The spacers separate the rails from the interior surface of the two ends of the oval tube. A hydraulic jacking mechanism is inserted into the bore of the rail gun between the two rails, and force is applied to spread the rails apart (radially of the bore) and in this manner place the exterior filament-wound tube in tensile stress so as to preload the shell. While in this condition, a pair of rigid ceramic insulators is slidably inserted into the barrel to occupy the entire space between the two sets of rails and spacers except for the bore. When the jacking arrangement is withdrawn, the pretensioned filament-wound shell applies compression to the composite rail-insulator structure and preloads it.
The '179 patent teaches the manufacture of a gun barrel with a precompressed ceramic liner created by a tensioned overwrap in the form of an outer composite sleeve of braided tows of high tensile stress graphite fibers in a matrix of a thermosetting polyimide resin. An apparatus is shown for tensioning the graphite fiber braid while the structure is baked in an oven to cure the epoxy or polyimide resin that saturates the braided graphite fiber structure, providing the pretensioned shell for the gun barrel.
Although all of the above-mentioned concepts have focused upon the need to provide an arrangement for prestressing the bore components in order to place them in a state of compression which is higher in magnitude than the stresses that will be induced by the electromagnetic and/or plasma forces which will tend to move them radially outward, unfortunately, it is felt that none of these solutions has provided a truly lightweight structure wherein bore components are maintained in a constant state of hoop compression even while the interior is experiencing high internal forces, which may include repulsion forces that are tending to separate the two rails. Such forces may often exceed 10 MegaNewtons per meter of rail, which may be in addition to bursting pressure resulting from a plasma armature. As a result, the search has gone on for improved rail gun barrel assemblies.